FIG. 1 shows a known form of video coder. Video signals (commonly in digital form) are received at an input 1. A subtractor 2 forms the difference between the input and a predicted signal from a predictor 3 which is then further coded in box 4. The coding performed here is not material to the present invention, but may include thresholding (to suppress transmission of zero or minor differencs) quantisation or transform coding for example. The input to the predictor is the sum, formed in an adder 2 of the prediction and the coded difference signal decoded in a local decoder 6 (so that loss of information in the coding and decoing process is included in the predictor loop).
The differential coding is essentially inter-frame, and the predictor 3 could simply consist of a one-frame delay; as shown however a motion estimator 7 is also included. This comapres the frame of the picture being coded with the previous frame being supplied to the predictor. For each block of the current frame (into which the picture is regarded as divided) it identifies that region of the previous frame which the block most closely resembles. The vector difference in position between the identified region and the block in question is termed a motion vector (since it usually represents motion of an object within the scene depicted by the television picture) and is applied to the predictor to shift the identified region of the previous frame into the position of the relevant block in the current frame, thereby making the predictor output a better prediction. This results in the differences formed by the substractor 2 being, on average, smaller and permits the coder 4 to encode the picture using a lower bit rate than would otherwise be the case.
The motion estimator must typically compare each block with the corresponding block of the previous frame and regions positionally shifted from that block position; this involves a considerable amount of processing and often necessitates many accesses to stored versions of both frames.
The present invention is defined in the claims.